WASHINGTON -- A consumer action group organized by Ralph Nader called on Wednesday for more stringent government testing of home radon testing companies after its own independent study found that three of seven companies failed to accurately measure concentrations of the radioactive gas.

The group, Public Citizen, said that some of the false readings were too high. Others were too low. In somes cases, the unwary homeowner could wind up making unnecessary and expensive repairs to lower non-existent radon levels, or be lulled into a false sense of security, the group said.

``Those tests show that an alarming number of radon test kits currently on the market may not be doing the job,`` Public Citizen president Joan Claybrook told a news conference.

The seven firms account for about 70 percent of all home radon kits sold nationwide. Three of the seven companies the group said had failed its tests immediately challenged the findings and said that the Public Citizen study was unscientific and unprofessional.

Radon is the nation`s second leading cause of lung cancer. Smoking is first. The National Academy of Sciences estimates radon accounts for 13,000 deaths annually. Because radon is believed to compound the harmful effects of smoking, the academy has said that most of those deaths are among smokers.

The findings from their yearlong but limited study, Claybrook said, point up the need for the Environmental Protection Agency to impose more stingent tests before recommending testing companies to the public. She said all seven companies had previously passed EPA tests.

About 850 companies have been placed on the EPA`s ``approved list`` after the EPA testing. But, the agency does not actually certify the companies and has no plans to seek congressional authorization to do so.

The EPA admitted on Wednesday its testing procedures were flawed and announced that a limited number of more rigorous tests like those undertaken by Public Citizen would begin next spring.

Ned Doyle, a spokesman for Air Chek, an Arden, N.C., company that failed one of Public Citizen`s two testing rounds, vigorously disputed the findings. `I have never seen a program run so poorly, so unscientifically, supposedly in the public interest. This is staggering,`` he said.

Spokesmen for the other two companies that failed at least one of Public Citizen`s two tests -- Douglas Martin & Associates of Emmaus, Pa., and Barringer Laboratories Inc. of Golden, Colo. -- voiced similar views. Both said that their companies had not only been repeatedly approved by the EPA, but the accuracy of their test kits had been verified by customers who conducted their own quality assurance tests.